When I made the 2.0 key, I tried two different types of springs. I have these springs in my room somewhere. I believe one type is better than the other because of the material it is made of. My design was innovative at the time, and people that have keys (that you've tried) probably duplicated my design at least in part. My design was available from Shapeways, and you could even 3D rotate my design on their website, so copies were bound to pop up.
My design of the key was publicly available, but I never revealed exactly which springs I used.
I'll have to check if the type of springs that I found worked best, are still available.